Gas turbine engines may be used to power various types of vehicles and systems, such as air or land-based vehicles. In typical gas turbine engines, compressed air generated by axial and/or radial compressors is mixed with fuel and burned, and the expanding hot combustion gases are directed along a flowpath and through a turbine nozzle having stationary vanes. The combustion gas flow deflects off of the vanes and impinges upon turbine blades of a turbine rotor. A rotatable turbine disk or wheel, from which the turbine blades extend, spins at high speeds to produce power. Gas turbine engines used in aircraft use the power to draw more air into the engine and to pass high velocity combustion gas out of the gas turbine aft end to produce a forward thrust. Other gas turbine engines may use the power to turn a propeller or an electrical generator.
Compressor discharge (T3) and turbine inlet (T4.1) temperatures continue to rise (advanced gas turbine engines will significantly benefit from compressor discharge temperatures in the 1,200-1,400° F. range with turbine inlet temperatures well above 3,000° F.) to enable improved engine cycle efficiencies. Increased temperatures at the turbine disk rim and stress/temperature combinations well above the metallurgical limit of the conventional turbine rotor result, limiting turbine rotor life and hindering engine cycle improvement options. For example, a conventional turbine rotor using an insertable fir-tree design for attaching the turbine blades of a single crystal (SC) alloy to the turbine disk of a powder metal (PM) alloy may not be strong enough to sustain high speed stresses at the higher T3 and T4.1 temperatures, and the turbine disk may not be capable of attaining adequate life at rim temperatures above 1,300 to 1,400° F. In addition, such conventional turbine rotors are expensive to manufacture, susceptible to detachment or separation of the turbine disk due to high stress/temperature combinations, and there is a potential for turbine blade walking (axial dislocation of the turbine blade in the disk).
Another conventional turbine rotor includes individual SC turbine blades brazed or diffusion bonded together to form a blade ring that is subsequently brazed or diffusion bonded to the PM turbine disk. There are inherent metallurgical problems with brazing or diffusion bonding the SC turbine blades to the PM turbine disk as these bonding techniques are performed at high temperatures and may compromise the microstructure of the turbine disk. In addition, the multiple high temperature thermal cycles from bonding and heat treatment will likely result in grain growth in the turbine disk alloy, thus compromising low cycle fatigue (LCF) behavior in the disk hub. Diffusion bonding can also lead to the formation of deleterious interface phases from the diffusion of elements from one alloy to the other from several hours of high temperature exposure. Bond plane phases can be distributed in such a manner as to compromise strength and toughness. Conventional diffusion bonding or brazed approaches for attaching the turbine blades to the turbine disk in the conventional turbine rotor may also form carbides at the SC to PM bond plane, leading to a brittle bond joint and a subsequent reduced allowable design temperature and stress.
Moreover, diffusion bonding may require a high temperature vacuum environment. For diffusion bonding the turbine blades in an economical way, the turbine blades may be bonded simultaneously in a vacuum furnace. However, the mechanical loading on the turbine blades to press them into the turbine disk and hold them securely during simultaneous bonding is a challenging undertaking. The forces required for the diffusion bonding are very large, the tooling is expensive, and the resulting bonding may cause distortion in the resulting turbine rotor. In addition, a diffusion bonding process may compromise protective coatings on the turbine blades, so any protective coatings must be applied to the turbine blades in a potentially difficult non-line of sight process after diffusion bonding.
Hence, there is a need for hybrid bonded turbine rotors and methods for manufacturing the same. There is also a need for hybrid bonded turbine rotors that can withstand higher compressor discharge (T3) and turbine inlet (T4.1) temperatures, enabling improved engine cycle efficiencies and turbine rotor life, thereby resulting in reductions in specific fuel consumption and turbine rotor weight and cost. There is an additional need for methods for manufacturing the hybrid bonded turbine rotors in which suitable metallurgical properties of the turbine disk and turbine blades are maintained.